Sneaking may be one of the most important skills in Fallout 3 for certain characters. Sneaking up on a target allows the player character to get closer, allowing them to avoid taking damage for the longest time possible, and maximizing the damage potential through the use of "sneak-attacks." Sneaking increases the chance to loot and steal, and allows the ability to avoid some undesired confrontations, thus working as an offensive and defensive skill.

Unlike its predecessors, in Fallout 3 characters can both sneak and run at the same time (i.e. move faster while crouching) regardless of perks. However, without the Silent Running perk, they will suffer a detection penalty while running.

The sneaking state is displayed while crouching, and alerts the player character to detection and possible detection.

[HIDDEN] means being undetected. In this state, attacking a target guarantees the sneak attack critical bonus, pickpocketing is available, and it is safe to steal, lockpick, or otherwise access "owned" property.

[DETECTED] means being in sight by one or more targets. Pickpocketing is enabled, but almost always fails. Attempting to steal, lockpick, or otherwise access "owned" property will fail, and may result in friendlies turning hostile or confiscating stolen goods. Sometimes an unfriendly target will see it and this will be shown instead of [CAUTION], because they haven't identified one as hostile or a target.

[CAUTION] means one or more unfriendly targets are suspicious of presence and are searching for the character. After a time the enemy will dismiss the disruption and the state will return to [HIDDEN]. Pickpocketing is disabled for the enemy searching for the PC. Attacking in this state will not cause a sneak-critical hit.

[DANGER] means hostile NPCs are aware of one's presence, but they do not know the current location. They will actively search at or around last known location. If [DANGER] is flashing, it means that the enemy has the PC in sight and will engage them. The state can be returned to [HIDDEN] by moving away and staying out of sight for a long period of time. Note: As soon as the conditions are satisfied for any state it replaces the one above it on the HUD even though the PC may still be in the [HIDDEN] state to several targets around them. Essentially, it reports the worst detection state towards the character of all potential targets. Once all targets that satisfy the most-elevated state are dead (as in killing everyone actively pursuing the target in [DANGER]), the state will revert to the highest remaining state amongst the remaining targets.

Sneak skill: The sneak skill applies whenever the player character crouches. The higher the Sneak skill, the easier it is to remain undetected while attacking, stealing, or picking someone’s pocket.

Cloaking fields: The Stealth Boy and Chinese stealth armorOperation: Anchorage (add-on) will wrap the user in a cloaking field that makes them more difficult to detect while sneaking, rendering the character virtually undetectable by visual sighting. The stealth field effect is extremely powerful, and its value matters very little, so the Chinese stealth armor's stealth field of 5 works almost (but not quite) as well as the Stealth Boy's 75. It should be noted that this field only increases one's sneaking ability, and the field will be negated if leaving sneak mode in any conventional manner, such as firing a loud gun, since the cloaking field does not mask sound-based detection. The stealth suit only has a +15 Sneak versus the Stealth Boy's +100, but its effects are not limited by time. Cloaking can also be stacked, so the stealth armor and Stealth Boy can give a PC a combined field of 80.

Enemy's Perception: Enemies with low Perception are easier to sneak around than enemies with high Perception. A target's Perception will be impaired (a -4 reduction) if they have a crippled head.

Line of sight: While out of sight an enemy can only detect the target through sound.

Light level: The target is more difficult to see in dark areas or at night if outdoors. If the Pip-Boy light is active, it will give away the position.

Distance: The farther away the target is from the seeker(s) the less likely they would be detected.

Movement: The player character is hardest to detect when motionless and easiest to detect when running. One can avoid the sound aspect of this by the Silent Running perk. Moving faster while within line of sight will still make it easier to be seen, but one will no longer get any penalty for movement speed to sound made. NOTE: Looking around counts as movement, but not as much as movement over ground. If standing in shadows with a direct line of sight to a target not aware of the location, looking around may very well make the target investigate, or immediately end with getting one found.

Contact: Touching or running into targets while crouched will immediately alert them of position. Regardless of how hidden one is, bumping into a target, friendly or otherwise, will immediately result in detection. This also applies to robots that have been deactivated using the Robotics Expert perk.

Equipped weight: The total weight of all equipped items (weapons count only if they are in your hands) makes footstep volume louder for non-player characters. Silent Running removes this penalty entirely, allowing full sneaking ability in any equipment. The footsteps are still audible while sneaking with the perk, but non-player characters will not hear it.

Sound: Most guns make noise. Louder guns makes one easier to detect. The silenced 10mm pistol, BB gun, dart gun, InfiltratorThe Pitt (add-on), PerforatorThe Pitt (add-on), all melee weapons and all unarmed weapons are considered entirely silent, and thus will not break sneak mode. Grenades will also not break sneak mode, as they only make a noise at where they explode. Hitting but not killing an enemy with silent weapons still causes a [CAUTION] alert. All other weapons will most likely be heard, depending on one's distance from the target. Missile launchers and Fat Man units make noise at the location and then at the impact point as well. Oddly enough, the Pip-Boy's radio has no effect on sneaking.

Dead Bodies: When an enemy spots the dead body of another enemy, it will often enter the [CAUTION] state and move toward the dead body. This can often create a more or less perpetual [CAUTION] state while the player remains in the area. If a dead enemy can be grabbed and dragged out of the way before its friends find it, this can help the player retain the hidden state.

A Sneak Attack Critical occurs in Fallout 3 and Fallout: New Vegas when attacking while [HIDDEN]. Successfully hitting the target will result in a guaranteed critical strike (unless the weapon does not allow critical hits), while at the same time doubling (ranged) or quintupling (melee/unarmed) total damage.

While sneaking and [HIDDEN] using the "activate" function while facing a non-player character (when in touching range) will allow pickpocketing them; in other words, steal items that they are personally "holding." This will open the inventories of both the player character and the target. Items can be transferred either way, and thus items can also be planted on them (also known as reverse-pickpocketing, or the "Shady Sands Shuffle").

For each item the player character steals or plants in a non-player character's inventory, there is a chance of being detected (presumably based on the player character's Sneak skill, Luck, item weight/value and the non-player character's Perception). If detected, the non-player character will automatically reclaim the item(s) taken and the player character will not be able to pickpocket that non-player character again (at least not straight away - leaving the area and returning 24 hours later should allow the player character to re-attempt this).

The player character cannot steal items a non-player character currently has equipped. To steal an item the non-player character is wearing or using (armor or weapon), the player character can reverse-pickpocket a better item (higher DR for armor, higher damage value for weapons) into their inventory and then leave and re-enter the area. The non-player character will equip the item with the highest stats and then the player character may try to steal the item they were using before. For melee weapons, reverse-pickpocket a better ranged weapon onto the non-player character along with the appropriate ammunition. When the non-player character is reloaded, it will instead equip that new weapon. Steal the melee weapon and the ammunition they were given, and reload the non-player character. The non-player character will be unarmed and the player character can steal the ranged weapon back. Keep in mind that just because the player character can do more damage with a specific ranged weapon doesn't mean that the non-player character will (usually but not always). Experiment to get the desired effect.

Successfully stealing the ammunition from an non-player character with a gun equipped will immediately render that non-player character unable to fire their gun. The amount of ammunition an non-player character is carrying will not match the amount they could fire if not pickpocketed (which is actually unlimited as long as they have 1 round for that weapon); this discrepancy can be attributed to gameplay purposes. This tactic can be used to influence the outcome of both non-player character/PC and non-player character/Non-player character conflicts. Concerning non-player character/Non-player character conflicts, this allows remaining non-hostile to non-player characters on both sides of a conflict (by not needing to intervene with gunfire) while still ensuring victory for one side or another. Mind that after some time, possibly the general respawn period of 72h game time, ammunition will be restored on NPCs.

Non-player characters will notice the pickpocketing of other non-player characters, hence the [DETECTED] sneak state, but in large open areas one can maneuver into gaps or around corners to avoid line-of-sight and return to [HIDDEN]. Non-player characters lying in beds may appear to have their back in one's direction, but still have one in sight, this is because as far as the game is concerned they are not facing the way they are lying down, that is merely an animation. To get "behind" an non-player character lying in bed, go to the foot or head of the bed.

The player character can also use pickpocketing offensively to plant a live explosive. This will detonate after a short time, automatically killing the target regardless of the PC's Explosives skill or the target's health or resistances.

The Mothership Zeta add-on added another way to pickpocket a friendly non-player character. Reverse-pickpocket a cryo mine or cryo grenade onto the non-player character. After exiting the pickpocket menu, the cryo charge will go off but only stun or knockdown the non-player character. The non-player character can then be pickpocketed like any disabled target, with the exception that the non-player character won't turn hostile when they recover.

Picking up non-player character-owned items (ie. items whose text name is red) in the game world while sneaking and [HIDDEN] will allow stealing items undetected, though Karma penalties may still apply.

Using the Paralyzing Palm, one can pickpocket non-player characters regardless of whether being detected or not. Thus even an alert, attacking enemy can be paralyzed and robbed.

Pickpocketing from "good" or "neutral" characters will result in a karma loss.

Though silenced guns make no noise, if they impact near a target, they will investigate the impact site as if they're trying to figure out where it came from. Using this, one can easily lure enemies towards traps or other dangers or away from places they're guarding.

It should be noted that when entering third-person perspective, one can peek around corners without being seen. This minor exploit can be useful sometimes when sneaking to see what type of enemies are around corners or if there are any traps.

Be careful when sneaking in dark areas or at night, as it is possible to accidentally step on a tripwire, pressure plate or landmine. This can be avoided with the Light Step perk, or having a high enough Sneak skill and a stealth field to counteract the effects of the Pip-Boy light being turned on.

The player character can pickpocket in plain sight if the victim is "fleeing."

It is not a hostile act to look in people's pockets or look in an owned container and not take anything. This can be useful if having second thoughts about pickpocketing or stealing.

Barrels, boxes and other objects can be used to block NPC's line of sight. This is particularly effective in Megaton due to its numerous barrels.

The PC can "grab" an item (gun, chem, food, etc.) by clicking the right thumb stick (Z key on the PC) and then carry it to a secluded area and then steal it.

Items stolen from shop owners cannot be sold back to the original shop, but will not be removed from inventory if trying to do so. The items can be sold at any other shop.

Stealing from "good" or "neutral" characters will always result in a Karma loss. Stealing from "evil" characters will sometimes affect karma, and sometimes not. An example of an evil character from which one can steal from without losing karma is Smiling Jack in Evergreen Mills, and Eulogy Jones in Paradise Falls. Some examples of evil characters that will negatively affect karma when stolen from are the cannibals of Andale and the Chinese soldiers of various rank found in the Operation: Anchorage DLC simulation.

A stealth field can be created through the console by using the command player.setAV Chameleon xx. This is the same way the character is cloaked in the Elder Scrolls series, which also uses the Gamebryo engine. Unlike in the Elder Scrolls, Chameleon does not create any visual effect on the PC on-screen, but the effect persists.

Items without weight or value, such as spent ammunition casings (e.g. Case 9mm, Drained microfusion cell, etc.) can be pickpocketed without a chance of failure or karma loss, although this does not count as a "Picked Pocket" for any achievements. Many keys and cards also fall in this category.

For items with weight or value, a pickpocket attempt has a minimum chance to succeed of 5% and a maximum chance of 85%.

A failed pickpocketing attempt is not guaranteed to turn NPCs hostile, but it is possible.

When hidden by a Stealth Boy in enemy territory, they can be pickpocketed.

Stealing ammunition will make a non-player character unequip their weapon due to not having the ammunition for it, allowing even that to be stolen.

If an item is in a difficult spot to steal but is not in an inventory of some kind (wardrobe, desk, person, etc.), one can easily steal it by the "Grab" function and taking it somewhere out of sight. NPCs do not perceive grabbing as stealing so they will not react unless when stealing the item the player character is still in their sight.